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Hello everyone, Thank you so much for such a constructive information and pointing out some useful resources. I couldn't able to work on this due to deadline of other projects. Sorry for not being active in the thread that I created, and often I am out office too. I will definitely try to incorporate all possible ideas together at my understanding. I really appreciated everyone's suggestions. I will write back to this thread once I work on this project. With lots of thanks! Milan On Wed, Nov 12, 2014 at 9:57 AM, Andy Anderson <[log in to unmask]> wrote: > Nice to see this built-in to PostGIS’ geography type: > > “By default, all distance and area calculations are done on the > spheroid. You should find that the results of calculations in local areas > match up will with local planar results in good local projections. Over > larger areas, the spheroidal calculations will be more accurate than any > calculation done on a projected plane. All the geography functions have the > option of using a sphere calculation, by setting a final boolean parameter > to 'FALSE'. This will somewhat speed up calculations, particularly for > cases where the geometries are very simple.” > > http://postgis.net/docs/using_postgis_dbmanagement.html#PostGIS_Geography > > (They need to adjust their documentation, however; further down they say > “great circle” when they really mean “geodesic”.) > > ArcGIS seems to use chordal distances for almost everything: > > “When your input features are not projected (i.e., when coordinates are > given in degrees, minutes, and seconds) or when the output coordinate > system is set to a Geographic Coordinate System, or when you specify an > output feature class path to a feature dataset that has a Geographic > Coordinate System spatial reference, distances will be computed using > chordal measurements and the Distance Method parameter will be disabled. > Chordal distance measurements are used because they can be computed quickly > and provide very good estimates of true geodesic distances, at least for > points within about thirty degrees of each other. Chordal distances are > based on a sphere rather than the true oblate ellipsoid shape of the earth. > Given any two points on the earth's surface, the chordal distance between > them is the length of a line, passing through the three dimensional earth, > to connect those two points. Chordal distances are reported in meters.” > > > http://resources.arcgis.com/en/help/main/10.2/index.html#//005p00000005000000#GUID-ABBD96AC-100C-4AA6-9630-8D2A0507166F > > The buffer tool is an exception, however: > > > http://resources.arcgis.com/en/help/main/10.2/index.html#//00080000001s000000 > > — Andy > > On Nov 12, 2014, at 8:41 AM, Bistrais, Bob <[log in to unmask]> > wrote: > > I suggest looking into the Geography data type in your spatial database > (PostGIS supports it, not sure about spatial databases). The Geography > type is based on WGS84 (SRID 4326), and it sounds like this is what you > need for this exercise. > > > ------------------------------ > *From:* Northeast Arc Users Group [[log in to unmask]] on behalf > of Czaja, Daniel [[log in to unmask]] > *Sent:* Wednesday, November 12, 2014 8:17 AM > *To:* [log in to unmask] > *Subject:* Re: Finding Distance between points over water > > While I have no idea how to do this, it is a fascinating topic. It > conjures up images of sailing ships plying the oceans along great circle > routes in search of spices and treasure. > > > The one thing that I did find was that the NOAA and the Department of > Commerce put out a publication that contains the distances between US ports > here: http://www.nauticalcharts.noaa.gov/nsd/distances-ports/distances.pdf. > Maybe they could be of assistance. > > > Dan > > > Daniel Czaja, GISP > GIS Coordinator > State of Connecticut Department of Emergency Services and Public Protection > > Division of Statewide Emergency Telecommunications > 1111 Country Club Road > Middletown, CT 06457-9294 > > > Phone: 860.685.8131 > Fax: 860.685.8363 > > > [log in to unmask] > > > > > > *"Give me a stopwatch and a map, and I'll fly the Alps in a plane with no > windows. If the map is accurate enough." - The Hunt for the Red October* > > > *From:* Northeast Arc Users Group [mailto:[log in to unmask] > <[log in to unmask]>] *On Behalf Of *Don Cooke > *Sent:* Tuesday, November 11, 2014 12:15 PM > *To:* [log in to unmask] > *Subject:* Re: Finding Distance between points over water > > > A friend at GDT suggested a projection-free way to calculate distances > between lat/long pairs: calculate the spherical angle between the two > points relative to the center of the earth, then convert the angle to a > distance using an assumed radius of the earth. I guess you need to assume > a spherical earth…. > > > Interesting way to think about it. We do tend to get the rope wound > around the axle given our comfort with projections. > > > ****** Don > > *From:* Northeast Arc Users Group [mailto:[log in to unmask] > <[log in to unmask]>] *On Behalf Of *Andy Anderson > *Sent:* Tuesday, November 11, 2014 8:29 AM > *To:* [log in to unmask] > *Subject:* Re: Finding Distance between points over water > > > No single projection will work, you must use different ones over subglobal > distances, which prevents a single calculation. But one approach would be > to use the plate carrée projection (your attached example) to generate a > rough route, then choose an appropriate projection over each leg, e.g. > two-point equidistant, and then project to a new raster and redetermine the > legs and then calculate the distance. Still won’t be the best possible > distance because you’re fixing points, but it should be a better estimate. > > > For the resolution you are using (~70 Km/pixel) I don’t think a land > buffer is necessary. > > > — Andy > > > On Nov 11, 2014, at 10:56 AM, Milan Budhathoki <[log in to unmask]> > wrote: > > > Hi Chris and Andy, > > > Thank you for your suggestions. I was given just Lat/Long of ship's > starting port and destination port in .CSV. Beside these I don't I have any > additional information. Yes, prevailing winds and ocean currents will play > a role in ships actual path but I think my collaborator is looking for the > shortest distance just over water regardless of any other assumption that > might affect on actual route. I haven't research at published route > documents but this is worth trying. Projection is another issue since > dataset has thousands of ports around the globe. I am thinking of the > "Merrcator Projection ?". I did a quick cost-path analysis for one of the > ports pair. Due to lack of information to make a cost-raster ( I used just > land and water mask as a cost raster) path tends to grip through land as > below; > > > <image84 Nov. 11.jpg> > > > > > > > > Here voyages tends to pass very closely to land (mostly touches land > mask). I am thinking to buffer land mask outward with X miles so that ships > path will be X miles way from land. Also I am considering to bring ocean > depth as another cost-raster if data is publicly available. Eventually, I > am write a python script or run model builder once I figure out better way. > > > > I would like to hear more if anyone has any thoughts ! > > > Thank you ! > > > > > > On Mon, Nov 10, 2014 at 11:38 AM, Andy Anderson <[log in to unmask]> > wrote: > > To remove the tedium, write a Python script. > > > Remember, though, that distance calculations depend on the projection you > use. If you want the cost-path from, say, New York to Sydney, there is no > single projection that will give you an accurate measure. > > > A better approach might be to determine the standard shipping lanes, > calculate the distances (if you can’t find them in a table), and piece > together routes. > > > More generally, you could set up an iterative algorithm to calculate > distances using spheroid-based angular calculations with restrictions based > on open water (e.g. at 40° north latitude, longitude will be restricted to > roughly –74° to –9° and 128° to 140° and 142° to -124°). > > > — Andy > > > On Nov 10, 2014, at 9:13 AM, Milan Budhathoki <[log in to unmask]> > wrote: > > > Looks like email that I sent yesterday didn't go through. > > Here it is again: > > > Hello Listserv, > > > I have point dataset of ship trips from one port to another. I want to > calculate the shortest distance between each port pair *over water*. > There are thousands of voyages, and 5,000 unique ports from all over the > world. One of the approach I can use in ArcGIS is to run the Cost-Path > tool having water/land as a cost raster to make a path only on water. But I > assume that the Cost-Path approach would be little tedious for a large > dataset. I wonder if anyone in this forum has a suggestion to calculate a > shortest distance between two points having restricted path. > > > I will highly appreciate your feedback. > > > > -- > > Milan Budhathoki > > > ------------------------------------------------------------------------- > This list (NEARC-L) is an unmoderated discussion list for all NEARC Users. > > If you no longer wish to receive e-mail from this list, you can remove > yourself by going to http://listserv.uconn.edu/nearc-l.html. > > > > ------------------------------------------------------------------------- > This list (NEARC-L) is an unmoderated discussion list for all NEARC Users. > > If you no longer wish to receive e-mail from this list, you can remove > yourself by going to http://listserv.uconn.edu/nearc-l.html. > > > > > > -- > > Milan Budhathoki > > > > -- Milan Budhathoki ------------------------------------------------------------------------- This list (NEARC-L) is an unmoderated discussion list for all NEARC Users. 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